A conventional liquid pump usually comprises a motor and a pump combined together to have a common shaft, and the housing of the motor and the pump base are made of cast iron. The motor is cooled by a fan fixed on its shaft and a fan of the pump is cooled by liquid in the pump. But if the liquid is stopped to supply to the pump, the pump fan and the motor are liable to burn up because of continuous operation. Three methods are used in a conventional liquid pump to prevent such an occasion from happening.
One method is to provide a temperature switch for the coil of the motor to cut off automatically electric power in case of the temperature of the coil exceeding a pre-set value, and to turn on once the temperature of the coil drops below the pre-set value. This protective way performed by the temperature switch cannot fully operate its function before the pump continues to operate for 15-30 minutes, so the insulation of the coil can be easily imparted and the pump fan can also be broken.
Another method is to provide at the entrance of the liquid a pressure valve comprising a metal resilient piece for sensing the pressure of the liquid. But if the liquid is supplied and stopped irregularly and alternately, the motor can also operate and stop in the same way, not good for the motor. In addition, the metal resilient piece may be adhered with miscellaneous material in the liquid to reduce its effectiveness.
The last method is to control operation of the motor by means of the difference of the current of the motor between loaded condition and unloaded condition so that the motor is turned off in case of no load. But the motor has to be turned on manually, unable to be turned on automatically, when the liquid begins to supply again.